“Charge comparison protection of transmission lines—relaying concepts” (Ernst, Hinman, Quam, Thorp; IEEE Transaction on Power Delivery, Vol. 7, No. 4, October 1992, pages 1834 to 1846) discloses a method of providing a fault signal. In this method, the respective current at each conductor end of a conductor is sampled forming current samples. The samples are in this case recorded at a time interval of 0.5 ms. The samples obtained in this case are integrated on a conductor-end specific basis to form charge measurement values, with the integration duration corresponding to half the period duration of the current—the method relates to an alternating current with a fundamental frequency of 60 Hz. The integration intervals in this case always start and end at zero crossings of the alternating current. The charge measurement values formed in this way are added up, to be precise firstly to form a scalar sum (sum of absolute magnitudes) by adding up the charge measurement values without taking into account the respective mathematical signs—that is to say without taking account of the charge flow—and secondly to form an arithmetic sum (absolute magnitude of the sum of the signed magnitudes)—referred to in the following text here as the overall charge measurement value—by adding up the charge measurement values taking into account the respective mathematical signs. The fault signal is produced when the overall charge measurement value (arithmetic sum) exceeds a threshold value which is dependent on the scalar sum, that is to say a threshold value which is matched to the respective measurement situation.
U.S. Pat. No. 4,939,617 also discloses producing a fault singal. In this method, a fault signal is produced when an internal fault occurs on a power transmission line. For this purpose, current measurement values are recorded at each of the two ends of the conductor and are integrated over a time period of one half cycle. The charge measurement values recorded in this way are transmitted via a communication line between the charge measurement appliances in both directions from the first charge measurement appliance to the second charge measurement appliance and vice versa.
A restraint value is in each case formed in evaluation devices which are associated with the respective measurement appliances, as the sum of the charge measurement value from the first measurement appliance and of the charge measurement value from the second measurement appliance, by adding the measurement values without taking account of their respective mathematical signs. Furthermore, a comparison value (operate value) is formed as a sum by addition of the two charge measurement values with the correct mathematical signs. This comparison value is multiplied by a factor, for example 3, forming a weighted comparison value. A fault signal which indicates an internal fault on the power transmission line is produced if the comparison value assumes a value which is greater than the restraint value.